Immune mediated platelet destruction occurs in idiopathic (immune) thrombocytopenia purpura (ITP), post transition purpura (PTP), neonatal isoimmune thrombocytopenia (NITP), and in other disease states. The antigens and antibodies involved in these conditions are not well characterized. Using techniques we have developed for identifying antigen bearing proteins of platelets, we will study the nature and structure of these platelet antigens and the antibodies which mediate platelet destruction. The technique involves electrophoretic separation of platelet membrane proteins, transfer to nitrocellulose paper, exposure to antibody, and detection of antibody binding using a radiolabeled second antibody. We propose to further characterize the PLA1 platelet antigen and to study the nature of the anti PLA1 antibodies in NITP. We will attempt to identify the platelet Fc receptor or Fab binding site to which platelet associated IgG may bind. Finally, we will further characterize the antigens and antibodies we have identified in a subject of children with ITP.